Eli Pariser gave an interesting presentation at TED named Beware online "filter bubbles". In short he goes through how when Google and Facebook use personalization to show you results, i.e. Google search results or Facebook status updates, it can be a form of censorship.
For example, Facebook shows you more information from your closer friends then others. They do not post all of your friends data on your home page because Facebook wants you to see things they think you find more interesting. Google is slowly incorporating social elements in search results and he also talks about how Google is using other means to personalize the search results (i.e. location, browser, language, web history, friends, etc.).
Watch the video - trust me - it will go by fast:
Do you think that this is a form of censorship? Take my poll:
Forum discussion at WebmasterWorld.